1. Field of the Invention
This invention relates to an automatic frequency control circuit utilized in an optical heterodyne receiver using a polarization diversity technique.
2. Background Information
In optical communication technology, for example, in high-capacity optical cable TV broadcasting technology, a coherent FDM broadcasting system has been studied. In such system, an optical heterodyne receiver unit with a polarization diversity technique is featured to reduce the influence of signal polarization fluctuation.
Further, according to the technique, it is very important to control accurately an oscillating frequency of a local light source device (i.e. a laser diode module) in the optical heterodyne receiver unit for heterodyne reception in order to obtain higher reception sensitivity and a broader tuning range. Therefore, there has been a need to provide a highly accurate automatic frequency control circuit.
FIG. 1 illustrates an example of a general block diagram of a conventional automatic frequency control circuit utilized in a coherent diversity receiver unit. This circuit comprises three main circuit portions, namely a first frequency detector portion 2-A including a first delay line 4-A and a first double balanced mixer 6-A, a second frequency detector portion 2-B including a second delay line 4-B and a second double balanced mixer 6-B, and an adder circuit 8. An output from the adder circuit is applied to a light source driver circuit 10 to control the frequency of a local light source device. (not shown)
In an optical heterodyne receiver using a polarization diversity technique, after receiving which have diversity signals having a different polarity from each other, a first intermediate frequency signal IF1 and a second intermediate frequency signal IF2 which are produced by respective receiver modules (not shown) are inputted to input terminals 12-A and 12-B of the automatic frequency circuit respectively.
Each of the intermediate frequency signals IF1 and IF2 is converted to a corresponding voltage signal by frequency detector portions 2-A and 2-B respectively, and the voltage signals are added to each other in the adder circuit 8. Because this circuit adds both intermediate frequency signals after conversions to corresponding voltage signals, it is possible to obtain a voltage signal output which is independent of the phase relationship between the original intermediate frequency signals IF1 and IF2.
Therefore, the output signal from the adder circuit 8 can be used as an error signal to control the local light source device (not shown) with a light source driver circuit 10 for heterodyne reception.
For example, in case the intermediate frequency signals IF1 and IF2 are modulated in PSK (Phase Shift Keying) form, a constant voltage signal can be obtained by inputting the intermediate frequency signals IF1 and IF2 to corresponding frequency detectors 2-A and 2-B after doubling each of the frequencies to recover the carrier frequencies using conventional doubler circuits.
However, the above conventional circuit has a disadvantage caused by a special characteristic of double balanced mixers.
FIG. 2 illustrates an example of a frequency-voltage conversion characteristic of a frequency detector using a double balanced mixer plotted for four different input signal powers. Assume that this frequency detector has a 1900 MHz zero-crossing frequency characteristic.
As shown in FIG. 2, if the inputted signal power is changed by the polarization status in a light wave path (i.e. in an optical glass fiber), the output voltage is also changed even if at the same frequencies. Therefore, the detection characteristic of the frequency detector is influenced by the polarization condition and it is difficult to maintain the stability of the above mentioned voltage signal output by the conventional automatic frequency control circuit under different polarization conditions.